The cut and paste is an old Unix thing. It's not just plan9.
The same (or very close) interface was available for unix in 1982 with the Blit graphics terminal. Blit was created by Rob Pike (acme creator) too and it influenced the next Pike's GUIs (rio and 8½).
In Plan9's rio, windows are treated as completely editable text, then the mouse chords for cut-and-paste works for all applications transparently.
Blit promotional video: https://www.youtube.com/watch?v=emh22gT5e9k
Wiki: https://en.wikipedia.org/wiki/Blit_(computer_terminal)